This invention relates to polyphosphazene polymers containing repeating ##STR2## units in the polymer chain in which one alkyl group per each three repeating units is attached directly to the phosphorus atom and to a method of preparing such polymers. More particularly, the invention relates to polyphosphazene polymers containing units represented by the structure: ##STR3## in which R is alkyl containing from 1 to 10 carbon atoms and X is chlorine or an --OR' radical (defined hereinafter) and to methods for the preparation of such polymers.
The polymerization of hexachlorocyclotriphosphazene, (NPCl.sub.2).sub.3, to poly(dichlorophosphazene), (NPCl.sub.2).sub.n, in which n is from 20 to 50,000, is a well-known reaction as illustrated by U.S. Pat. Nos. 3,370,020; 4,005,171 and 4,055,520. It constitutes the first step in the synthesis of stable poly(alkoxy- or aryloxyphosphazenes). [NP(OR).sub.2 ].sub.2, or poly(aminophosphazenes), [NP(NHR).sub.2 ].sub.n or [NP(NR.sub.2).sub.2 ].sub.n. These syntheses are accomplished by the utilization of (NPCl.sub.2).sub.n as a polymeric reactive intermediate for interactions with alkoxides, aryloxides, or amines. Poly(alkoxy or aryloxy phosphazenes) and poly(aminophosphazenes) and their method of preparation are described in the prior art as illustrated in the publications "Phosphorus-Nitrogen Compounds", Academic Press, New York, N.Y., 1972 by H. R. Allcock and "Poly(Organophosphazenes)", Chemtech, Sept. 19, 1975 by H. R. Allcock and in such patents as U.S. Pat. Nos. 3,515,688; 3,702,833; 3,856,712; and 4,042,561. A large number of high polymers have now been prepared by such techniques, and many of these have considerable fundamental and technological potential.
However, it has been recognized for some time that a further extension of this field to yield more-diverse polymers with specific properties (particularly high thermal stability) was dependent on the development of new synthesis routes that would allow alkyl or aryl groups to be linked directly to the phosphazene backbone through phosphorus-carbon bonds. The attainment of this goal has been the subject of intensive investigation in our laboratory, with an emphasis to date being placed on substitutive synthesis routes--i.e. on the reactions of organometallic reagents with poly(dihalophosphazenes). The alternative route--the polymerization of per-alkyl- or arylcyclophosphazenes, (NPR.sub.2).sub.3 --has so far proved to be unsatisfactory. However, earlier work by Allcock and Moore appearing in the publication "Macromolecules", Volume 8, page 377 (1975) suggested that phosphazene trimers that contained both phenyl and halogeno substituent groups could be converted to low molecular weight polymers, although the steric size of the phenyl groups appeared to be detrimental to the polymerization process.
More recently, a method for the preparation of mono-methyl-pentachlorocyclotriphosphazene has been described in the article entitled "Hydridocyclophosphazenes Synthesis Via Organocopper Reagents" by Harris and Allcock appearing in the Journal of The American Chemical Society, Volume 100, page 6512 (1978). As described in the aforementioned article, the compound is prepared by reacting hexachlorocyclotriphosphazene, (NPCl.sub.2).sub.3, with methyl magnesium chloride in tetrahydrofuran in the presence of [n-(C.sub.3 H.sub.7).sub.3 P CuI].sub.4 to first produce, after treatment with 2-propanol, a mono-methyl-hydridotetrachlorocyclotriphosphazene, following which this compound is reacted with chlorine.